The present invention relates to a semiconductor light emitting device and a fabrication method thereof.
Nitride based III–V compound semiconductors such as GaN, AlGaN and InGaN have forbidden band widths ranging from 1.8 eV to 6.2 eV, and therefore, they are theoretically capable of realizing light emitting devices allowing emission of light having wavelengths in a wide range from a long-wavelength side (for example, red) to a short-wavelength side (for example, ultraviolet). From this viewpoint, in recent years, such nitride based III–V compound semiconductors have become a focus of attention as materials for light emitting devices.
In the case of fabricating a light emitting diode (LED) or a semiconductor laser by using nitride based III–V compound semiconductors, it is required to stack a light emission layer (active layer), a first conductive type layer, and a second conductive type layer, which are made from nitride III–V compound semiconductors such as GaN, AlGaN, and InGaN in such a manner that the active layer is sandwiched between the first and second conductive type layers. In such a light emitting diode or a semiconductor laser, a light emission layer sometimes has an InGaN/GaN quantum well structure or InGaN/AlGaN quantum well structure.
The technique of forming a nitride semiconductor such as a gallium nitride based compound semiconductor by vapor-phase growth has an inconvenience that neither a substrate being lattice matching with the nitride semiconductor nor a substrate with a low dislocation density is present. To cope with such an inconvenience, a technique of reducing dislocations has been disclosed, for example, in Japanese Patent Laid-open No. Sho 63-188938 and Japanese Patent Publication No. Hei 8-8217. The technique includes the steps of depositing a low-temperature buffer layer made from AlN or AlxGa1−xN (x is 0 or more and less than 1) on the surface of a substrate such as a sapphire substrate at a low temperature of 900° C. or less, and growing a gallium nitride based compound semiconductor thereon, thereby reducing dislocations due to lattice mismatching.
The crystallinity and morphology of a gallium nitride based compound semiconductor layer can be improved by using such a technique.
Another technique for obtaining a high quality crystal having a lower dislocation density has been disclosed, for example, in Japanese Patent Laid-open Nos. Hei 10-312971 and Hei 11-251253. This technique includes the steps of stacking a first gallium nitride based compound semiconductor layer, forming a protective layer made from a material capable of prohibiting the growth of the gallium nitride based compound semiconductor such as silicon oxide or silicon nitride, and growing a second gallium nitride based compound semiconductor layer from a region not covered with the protective film in the in-plane direction (lateral direction), thereby preventing propagation of threading dislocations extending perpendicularly from the boundary with the substrate.
A further technique for reducing a threading dislocation density has been disclosed, for example, MRS Internet J. Nitride Semicond. Res. 4S1, G3. 38 (1999), or Journal of Crystal Growth 189/190 (1998) 83–86. This technique includes the steps of growing a first gallium nitride based compound semiconductor, selectively removing the film by using a reactive ion etching (hereinafter, referred to as “RIE”) apparatus, and selectively growing a second gallium nitride based compound semiconductor from the crystal remaining the growth apparatus, thereby reducing the threading dislocations density.
A crystal film having a dislocation density of about 106 cm−2 can be obtained by using these techniques, and a long lifetime laser can be realized by using such a crystal layer.
With the use of such a selective growth process, it is possible not only to reduce threading dislocations but also to fabricate a semiconductor device having a three-dimensional structure. For example, a three-dimensional semiconductor device structure can be obtained by forming an anti-growth film on a gallium nitride based compound semiconductor film or a substrate, and selectively growing crystal from an opening formed in the anti-growth film; or selectively removing the gallium nitride based compound semiconductor film or the substrate, and selectively growing crystal from the remaining crystal. Such a semiconductor device has a three-dimensional structure including a side plane composed of the facet and the apex (upper surface) continuous to the side plane, and is therefore advantageous in reducing damages in a device separation step, forming a current constriction structure of a laser, and improving the crystallinity by making effective use of the characteristic of the crystal plane composed of the facet.
FIG. 53 is a sectional view showing one example of a nitride based light emitting device having a three-dimensional shape formed by selective growth. In this example, the light emitting device is configured as a GaN based light emitting diode. As shown in the figure, an n-type GaN layer 102 is formed as an underlying growth layer on a sapphire substrate 101, a silicon oxide film 104 having an opening 103 is formed on the n-type GaN layer 102, and a hexagonal pyramid shaped GaN layer 105 is formed by selective growth from the opening 103 of the silicon oxide film 104.
The GaN layer 105 is a pyramid shaped growth layer covered with the S-plane, that is, the (1-101) plane if the principal plane of the sapphire substrate 101 is taken as the C-plane. The GaN layer 105 is doped with silicon. The tilt S-plane portion of the GaN layer 105 functions as a cladding portion. An InGaN layer 106 is formed as an active layer on the GaN layer 105 in such a manner as to cover the tilt S-plane, and an AlGaN layer and a magnesium-doped GaN layer 107 are formed on the InGaN layer 106.
In such a light emitting diode, a p-electrode 108 and an n-electrode 109 are formed. The p-electrode 108 is formed by vapor-depositing a metal material Ni/Pt/Au or Ni(Pd)/Pt/Au on the magnesium-doped GaN layer 107. The n-electrode 109 is formed by vapor-depositing a metal material Ti/Al/Pt/Au on a portion, exposed from an opening formed in the silicon oxide film 104, of the n-type GaN layer 102.
The above-described light emitting diode formed by selective growth, however, has a problem that the diode contains threading dislocations upwardly extending from the substrate. Another problem is that since the apex and/or the upper surface is surrounded by a side plane composed of a facet where the growth rate is low, the supply of a source gas becomes too large, tending to degrade the crystallinity of the apex and/or the upper surface. As a further problem, if the area of the apex and/or the upper surface is smaller than that of the substrate, it is difficult to control the film thickness and the composition of a mixed crystal at the apex and/or the upper surface. Accordingly, even if a semiconductor device having a three-dimensional structure is formed by selective growth, the crystallinity of the apex and/or the upper surface of the three-dimensional structure is degraded from the above reason, to cause various problems such as leakage of current due to the reduced efficiency by non-radiative recombination and irregular formation of PN junction.
In the case of forming an InGaN layer 106 as an active layer on a side plane of the three-dimensional semiconductor device by selective growth as shown in FIG. 53, an electrode can be formed only on the side plane by a specific electrode formation process; however, since it is difficult to keep the accuracy of photolithography for a three-dimensional structure, it is not easy to accurately form the electrode on the side plane, thereby reducing the yield. Also, in the case of forming an electrode on a side plane of the three-dimensional structure, a current tends to be propagated in a conductive layer being in contact with the electrode depending on the resistivity and the thickness of the conductive layer, with a result that the current is injected in the apex and/or the upper surface, thereby degrading the device characteristics.